1. Field of the Invention
The present invention relates to an image pickup lens mounted on imaging devices adopting relatively small and thin solid-state imaging elements such as CCD sensors and C-MOS sensors disposed on portable terminals such as cellular phones and smartphones, motion sensors for home-use game machines, and PDAs (Personal Digital Assistants).
2. Description of the Related Art
Recently, demands are increasing for further downsizing and thinning of portable terminals such as cellular phones and smartphones, or devices such as PDAs, and along therewith, there are demands for improving the camera function to correspond to the increased number of pixels. Therefore, further downsizing and increase in the number of pixels is realized in imaging elements adopted in the imaging devices mounted on such devices, and in general, image pickup lenses having a high resolving power corresponding to the increased number of pixels are required. However, in image pickup lenses adopted in sub-cameras of portable terminals or the like where the users of portable terminals use the camera to take pictures of themselves from a close distance, it is required that the image pickup lenses are further downsized, thinned, and capable of taking images of objects in a wide area, in other words, that the lenses have a wide angle of field. In addition, it is required that the lenses have sufficient resolving power so that the image quality of the scenery that is taken into the images by the lenses having a wide angle of field is good. Moreover, along with the downsizing of imaging elements, the illuminance sensitivity decreases, so that it is important that the lens system has a high brightness.
Conventionally, image pickup lenses configured of four or more lenses are preferable as the image pickup lenses having high resolving power to be mounted on above-described devices, but the increase in the number of lenses is not preferable from the viewpoint of downsizing and cost reduction. Further, image pickup lenses configured of one or two lenses are advantageous from the viewpoint of downsizing and cost reduction, but various aberrations are difficult to correct via a small number of lenses, so that there is a drawback that the required resolving power cannot be acquired. Image pickup lenses having a three-lens configuration have conventionally been proposed widely as a configuration capable of solving both above-mentioned problems to a certain extent. The three-lens configuration is also suitable for corresponding to the technological advancement of imaging devices where downsizing and thinning are further advanced.
Japanese Patent Laid-Open No. 2008-276200 (Patent document 1) discloses an image pickup lens adopting a three-lens configuration having, in order from an object side, a first lens of a positive meniscus shape with a convex surface facing the object side, a second lens with a concave surface facing the object side, and a third lens having a negative refractive power near an optical axis, wherein the second lens and the third lens respectively have at least one surface thereof formed as an aspherical surface.
Further, Japanese Patent Laid-Open No. 2007-206611 (patent document 2) discloses an image pickup lens having, in order from an object side toward an image side, an aperture stop, a first lens of a positive meniscus shape with a convex surface facing the object side, a second lens having a meniscus shape with a convex surface facing the image side, and a third lens having a meniscus shape with a convex surface facing the object side, wherein the first lens, the second lens and the third lens are each formed of plastic materials, the second lens having an object side surface formed as a concave surface, the third lens having a negative refractive power near an optical axis, and the second lens and the third lens each having at least one surface thereof formed as an aspherical surface.
Even further, Japanese Patent Laid-Open No. 2004-252312 (patent document 3) discloses an image pickup lens having, in order from an object side toward an image side, a stop, a first lens with a positive refractive power, a second lens with a meniscus shape having a weak positive refractive power and having a concave surface facing the object side, and a third lens with a negative refractive power and having at least one refractive surface formed as an aspherical shape.
Patent document 1 proposes an image pickup lens having a superior productivity, a compact design and a high image forming performance. However, the total track length of the image pickup lens is approximately 4 mm, which is not sufficient from the viewpoint of responding to downsizing and thinning. Moreover, since it has an F-value as high as approximately 3.5, if the lens is adopted in downsized imaging elements, an appropriate light quantity cannot be achieved at the circumference area of the imaging elements, and it may be necessary to increase the illuminance sensitivity of the lens system when taking images. However, noise is increased when the illuminance sensitivity is set high, and the image quality is deteriorated. Further, since the half angle of field of the disclosed lens is approximately 34 degrees, it does not have a sufficiently wide angle of field.
Patent document 2 proposes an image pickup lens capable of realizing downsizing and enhanced optical performances by reducing the back focus. However, the total track length of the disclosed image pickup lens is approximately 5 mm, and it does not correspond sufficiently to downsizing and thinning. Since the disclosed image pickup lens has a short back focus, if further downsizing and thinning is required, it becomes difficult to ensure enough space required for arranging insert objects such as infrared cut filters and cover glasses. Further, since the half angle of field of the image pickup lens is approximately 32 degrees, it does not have a sufficiently wide angle of field.
Patent document 3 proposes a high resolution and compact image pickup lens realizing cost reduction while considering the assembly accuracy. In the disclosed image pickup lens, the F-value is approximately 2.4, which realizes a relatively bright lens system that can be applied to imaging elements where downsizing is advanced. However, the total track length of the image pickup lens is approximately 6 mm, which is insufficient from the viewpoint of downsizing and thinning. Moreover, according to embodiments 1 and 2, the image pickup lens is expensive, since the first lens is formed of glass material. Even further, it is difficult to fabricate a lens having an extremely small thickness using glass material, so that there is a limitation to further thinning of the lens.